scope:

Scope of application

In Section 2 of the present guideline humid air and combustion gases are first considered as mixtures of ideal gases in equilibrium, but without taking dissociation effects into account. The following components are considered

• Nitrogen
• Oxygen
• Argon
• Neon
• Water
• Carbon dioxide
• Carbon monoxide
• Sulphur dioxide

With these assumptions, in the temperature range

200 K ≤ T ≤ 3300 K

Equations (10) to (21), empirical equations used for calculating caloric state variables, will apply without limitation of the pressure range. However, these basic assumptions will in practice lead to a considerable restriction in the scope of validity.

At temperatures above 1200 K, the caloric properties of combustion gases will be significantly affected by dissociation effects. These effects require the calculation of simultaneous equilibria which is both numerically complex and involves elaborate programming. In Section 3 of the present guideline a simplified model which takes dissociation effects into account is introduced; it delivers results of adequate accuracy but both numerically and from the programming point of view is a considerably simpler and permits higher computation speeds than exact models. Details of the scope of application of the simplified dissociation model will be found in Section 3.1 while details of the influence of dissociation effects can be found in Annex A.

It is particularly at low temperatures and high pressures that the real-gas behaviour of humid air and combustion gases is increasing in importance. These effects can usually be ignored in applications relating to guarantee data certification for gas-turbine installations. But if the calculation rules described in Section 2 for determining c_p^o, h^o and s^o are to be used for other applications, ignoring the real-gas influence may result in significant errors occurring. Information on the influence of real-gas effects is provided in Annex B.

For reasons related to process technology, falling below the dew-point of sulphuric acid is usually avoided. Although the position of the sulphuric acid dew-point has no relevance to the energy balances and efficiency calculations in guarantee certification,

Annex C provides approximation equations for determining the corresponding temperature.

If, as part of guarantee certification, exergetic efficiencies are to be calculated, it may then be necessary to take a liquid or a solid phase of water into account in calculating the enthalpy and entropy of the combustion gases in the ambient state. The corresponding calculating rules are summarized in Annex D.

Purpose of the guideline

The knowledge of the thermodynamic properties of working fluids in thermal machines or apparatus is a precondition of all calculations required in the design and adjustment of, in guarantee certification for, and in monitoring the machines or apparatus in question. Preparation of the guideline for the thermodynamic properties of humid air (in the compressor) and combustion gases (in the turbine) was based on calculations for gas turbines; the corresponding properties can, however, also be used for other components and processes and their associated gases provided the limits of the scopes of application are observed.

In practice, at least seven mathematical formulations, sometimes differing considerably from each other, have been used to date for the thermodynamic properties of air and combustion gases and persistently give reason for discussions and incorrect interpretations, cf. Annex E and [4]. In the light of the increasing importance of gas turbines to the power-supply industry such uncertainties should no longer be tolerated.

The aim of this new guideline will therefore be

• to reproduce as accurately as possible the thermodynamic behaviour of the process gases within the entire region of interest when taking into consideration the technological developments to be expected
• to demonstrate the influences of dissociation, of real-gas behaviour and of condensation which limit the accuracy of the simple models, and, where necessary, to include such influences
• to optimize the formulation of the equations with regard to accuracy and calculating speed
• by means of comparisons to demonstrate the shortcomings of popular physical property models so as to make users aware of the necessity for a common database of sufficient accuracy

Purpose of this guideline is not the estimation of the composition of gas mixtures. For the calculation of the composition of humid air and combustion gases an example may be found in VDI 2048-3 [19].